Heat storage is required in solar systems to buffer the fluctuations in and between energy collection and release. Heat storage also enables use of waste or surplus heat from industrial processes. Heat storage at low temperature is also required to allow the use of cooling machinery during times other than the time of use of the cooling capacity for space cooling, air conditioning and other purposes.
While heat may be stored by raising the temperature of the storage material, as in sensible heat storage, latent heat storage offers numerous advantages over sensible heat storage. The heat of transition in melting is generally greater than the heat capacity of a material integrated over a practical temperature range, hence a larger amount of energy can be stored in a given storage volume as latent heat of melting than as sensible heat. Additionally, latent heat storage systems provide heat at a constant output temperature, the transition temperature of the phase change, in contrast to a sensible heat storage system in which the output temperature decreases as heat is removed.
In a latent heat storage system, heat is added to the storage medium until it has undergone complete phase change. The transition most commonly used is solid to liquid. Systems based on the melting of salts or salt hydrates are convenient to work with and have high energy densities. Input heat is stored as heat of melting and additionally as sensible heat and, when heat is required, the melt, which may indefinitely be kept supercooled in the intermim, is nucleated and heat is removed as the liquid crystallizes.
A major problem in previously known systems of this type is the removal of heat from the crystallizing body. The use of conventional heat exchangers immersed in the solid-liquid phase change materials (hereinafter referred to as PCM) has been hampered in known systems by two significant problems. During the heat removal cycle, the solid crystallizes and coats the exchanger surfaces thus increasing their thermal resistance and decreasing the rate of heat transfer. Also, in the solid state, the materials may contract away from the heat exchanger surfaces which leads to a decrease in the initial rate of melting during the heating cycle. Efforts to solve these problems have included coating the heat exchange surfaces with surfactants and non-adhesive materials, such as "Teflon" without notable success. Attempts to modify the crystal habit of the PCM with additives to weaken the crystal aggregate have not provided a satisfactory remedy. And mechanical methods for clearing the heat exchange surfaces are cumbersome, energy demanding and have had only limited effectiveness.
Heat exchange by direct contact between the crystallizing PCM melt and an immiscible, non-volatile heat transfer liquid has been proposed to eliminate the need for a conventional heat exchanger. Silicone oil and other immiscible, non-volatile fluids are pumped through or swirled over the molten PCM and heat is removed as sensible heat of the fluid. This method suffers from the difficulty that the fluid stream may carry droplets and crystals of the PCM which clog the secondary heat exchange system. The method is also inefficient because of the unsatisfactory heat carrying capacity of the oil or other non-volatile heat transfer fluids.
A further heat exchange system has involved pumping a molten salt mixture at a temperature of from 250 degrees to 350 degrees C. into a boiler, injecting water into the molten salt mixture where it is flashed into steam by the molten salt and the steam is passed to a condenser/heat exchanger to deliver heat. The slurry of molten salt and salt crystals formed in delivering latent heat is pumped out for heating to melt the crystals in the slurry before return to the boiler. The system requires complicated and expensive apparatus and suffers from the further disadvantages that the steam tends to carry salt over to the condenser/heat exchanger surface, that the area of heat transfer from the salt to liquid water is limited to the water jets before conversion of the liquid water to steam, that the temperature range is unnecessarily high causing safety and implementation problems in high entropy heat applications, and that the system requires substantial mechanical energy. A further disadvantage is that in order to remain fluid enough for pumping and to allow passage of water and steam for delivery of heat, only a limited proportion of the salt in the boiler can be allowed to crystallize, thus limiting the effective energy density of the system. Finally, the method is not applicable to supercooling salt hydrate melt storage systems, but only to anhydrous salt melts at high temperature which inevitably lose their heat to the environment; only short term storage, associated with substantial heat losses, could thus be achieved if use of this method were attempted.